1. Field of the Invention
The present invention relates to a semiconductor output circuit having a protective measure against static electricity.
2. Description of Related Art
As disclosed in Japanese Patent Nos. 2000-077537, 2000-269435, and 2001-044291, for example, it is known to provide a protective circuit to a semiconductor output circuit that includes an insulated gate transistor such as a power MOSFET and IGBT, and a drive circuit applying a gate drive signal to the gate of the insulated gate transistor in order to improve resistance to ESD (electrostatic discharge) of the semiconductor output circuit.
FIG. 11 shows a structure of a typical semiconductor output circuit provided with such a protective circuit. As shown in this figure, this semiconductor output circuit 101 includes a MOS transistor 104, a drive circuit 106, a protective circuit 110, and external terminals 102, 103. The external terminals 102, 103 are connected to the drain and source of the MOS transistor 104, respectively. A resistor 105 is connected between the gate and source of the MOS transistor 104 for the purpose of pulling down the gate voltage to the ground potential. The protective circuit 110, an input of which is connected to the external terminal 102 and an output of which is connected to a node of the gate of the MOS transistor 104 and the resistor 105, includes a diode 109, a transistor 107, and a resistor 108. The output of the drive circuit 106 is connected to this node.
If a high voltage is applied between the terminals 102, 103 due to ESD (electrostatic discharge), the transistor 107 of the protective circuit 110 is turned on, and accordingly, the gate of the MOS transistor 104 is applied with the high voltage. As a consequence, the conductivity between the drain and source of the MOS transistor 104 increases to allow a surge current to flow therethrough. This action of the protective circuit 110 improves the resistance to ESD of the semiconductor output circuit 101.
However, as explained in detail later, in a case where a LDMOS transistor (Laterally Diffused MOS transistor) is used as the MOS transistor 104, it often occurs that the drain current (surge current) is concentrated on an outer portion of a cell area of the LDMOSFET when a high voltage is applied between the terminals 102, and 103, and accordingly, the semiconductor output circuit does not exhibit adequate resistance to static electricity.